2 On vocal biogeography and conservation: patterns of song traditions and
2.4 Discussion
2.4.3 Conservation implications and future avenues
Vocal divergence can affect effective communication among birds, as songs play a critical role in territory defense and mating contests. For example, song sharing with neighbours might offer social benefits, which favour reproductive success, such as song matching possibilities to interact with particular individuals (e.g., Beecher et al. 1997; Nelson et al. 2001; Leader et al. 2008). Therefore, song divergence, due to rapid vocal drift in isolated surviving populations, may prevent effective breeding within and between kōkako fragmented populations. The success of multi-sourced translocated populations may be limited by considerable song divergence. This vulnerability may be particularly important in other species which, like kokako, have specific habitat requirements, low dispersal abilities, and monogamous pair bonds.
Assortative mating after translocating kōkako from multiple sources appears to be prevalent (Rowe & Bell 2007; D. Bradley, Univ. of Waikato, unpublished data). This indicates that pairs from the same population are more likely to communicate effectively and interbreed. Therefore, considering song differences may be decisive in determining compatibility between populations and the viability of translocated and small populations of kōkako. For instance, simulated secondary contact between three recently separated populations (two translocated and a single source after 5-9 years of isolation) showed acoustic divergence and reduced between-population song sharing but no discrimination based on behavioural responses (see Chapter 4). However, in contrast to recently isolated populations, a similar experiment showed that the surviving populations of Rotoehu and Mapara respond significantly stronger to local versus foreign song
when simulating secondary contact (D. Bradley et al., Univ. of Waikato, unpublished data). Further research is needed to establish the effect of vocal divergence on behavioural responses among other surviving isolated populations that can effectively be used as sources for translocating individuals.
Finally, dispersal data support the idea that juveniles can learn songs after dispersal and thus will integrate more easily to a new population. Although alteration of song may also be possible for mature birds, they are likely to be less flexible (e.g., Doupe & Kuhl 1999), and this could pose difficulties for territory establishment and pair formation. This study suggests that active management should incorporate such behavioural considerations, to date largely left aside in management decisions.
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Chapter 3
Reductions in population size compromise song repertoires and complexity of a rare duetting songbird*
3I collected 80 % of the data and carried out 100% of the analysis and write-up.
LEM and JRW provided comments and advice.
*In preparation for publication bySandra V. Valderrama, Laura E. Molles, & Joseph R.
Waas
By Sandra V. Valderrama
Abstract
The North Island kōkako (Callaeas wilsoni) is an endangered New Zealand forest songbird living in territories year-round. Sedentary habits, poor dispersal abilities and being a predator-naive island species have made the birds especially vulnerable to massive habitat loss and introduced predators. Scattered
populations, in isolated conservation areas of various sizes, have been rescued from extinction and are currently recovering. Acoustic communication is crucial for mate choice and territory defense in kōkako but behavioural consequences of isolation are poorly understood despite their potential effect on population
viability. We examined demographic effects on key song attributes and tested the prediction that reductions in population size may compromise effective
communication. We analysed song repertoires and repertoire sharing, vocal performance, song sharing and syntactical characteristics within surviving and translocated populations. We found that decreases in population size correlate with decreases in population repertoire size, within-bout diversity of phrase types, and certainty in syntactical structure. Our results indicate that song traditions and delivery may crucially depend on high social interaction rates and thus population size. Furthermore, we reveal that song traditions in fragmented populations of rare kōkako resemble song microevolution in island colonization events.
Key words: North Island kōkako, song complexity and repertoires, small populations, fragmentation, behavioural conservation
3.1 Introduction
Conservation of rare species with low population sizes is central to protecting global biodiversity in an era of rapid species decline and extinction (e.g., Saunders et al. 1991). Small populations are known to be susceptible to stochasticity, loss of genetic flexibility, the accumulation of deleterious genes, and Allee effects, all of which result in reduced fitness and can cause extinction. However, less is known about how behavioural changes may influence the viability of small populations and the persistence of rare species in modified habitats (Blumstein & Fernandez-Juricic 2010).
Vocal behaviour and song traditions learned and culturally transmitted among individuals, can be affected by demographic changes within populations (Lynch 1996; Payne 1996). Reductions in population size have historically and naturally occurred when individuals of a species colonize a new area such as an island (i.e., a founding event), but can also occur when there is anthropogenic habitat disturbance (e.g., Pimm et al. 1988; Williamson 1989) which fragments and reduces available habitat. As a result, changes in song patterns may be linked to fragmentation and isolation effects as well as changes in population size and viability (Laiolo & Tella 2007; Laiolo et al. 2008; Briefer et al. 2010). Therefore, insight into divergence of song traditions in disturbed habitats requires knowledge of vocal patterns in local populations, as well as understanding the
correspondence between the attributes of vocal signalling systems and demographic factors, and ultimately between microevolutionary changes and conservation.
The North Island kōkako (Callaeas wilsoni) is an endangered species endemic to New Zealand. They are large songbirds (length c. 38 cm, weight 218-233g) that only persist in mature hardwood and podocarp forests with abundant tawa (Beilschmiedia tawa; e.g., Powlesland 1987; Best & Bellingham 1991) which remain in scattered isolated areas (Lavers 1978; Innes et al. 2006). They are monomorphic and monogamous, and pairs hold territories of up to 14 ha that they defend year-round (Innes et al. 2006). Kōkako are not only sedentary but also, like many remote-island birds, have poor flying abilities that limit their dispersal distance from natal territories to an average of 1.62 km, according to data obtained from four populations (J. Innes et al. unpubl. data in Sinclair et al. 2006).
In this study, we explore relationships between the size of isolated
populations of kōkako and song attributes that may be dependent on high rates of social interaction: song sharing, vocal performance (e.g., singing rates), and complexity. Song and element sharing is a consequence of social learning and communicating within and among groups (e.g., Brown & Farabaugh 1997; Molles
& Vehrencamp 2001; Todt & Naguib 2000) and allows more effective territorial defense for individual birds of many species (Beecher & Brenowitz 2005). Song performance and complexity have different components, but here we focus on repertoire size, repertoire sharing, and how song is used (i.e., temporal
characteristics of performance, versatility and syntactical characteristics). In larger populations, the need to vocally interact with many neighbours and floaters, in order to maintain territories and pair bonds, can result not only in local song similarity but also increased neighbour interaction that in conjunction with vocal drift may promote more complex songs due to improvisation and matching (Mundinger 1975; Bitterbaum & Baptista 1979).
Kōkako males and females sing and combine their songs into loud antiphonal duets, audible over long distances (c. 1.6 km, Maning 1960), with typically greater contributions by the male. However, repertoires of song elements are not sex-specific and the birds sing with immediate variety. Song sharing among neighbouring kōkako and the use of song in response to playback suggest that song has a function in territory defence through countersinging interactions, whilst flexibility in natural duetting song sequences may convey information about pair bonding and motivation (Molles & Waas 2006; Molles et al. 2006).
Therefore, the use of song repertoires and song complexity can potentially be directly affected by the amount of social interaction as regulated by population size. Overall territory distribution of kōkako and patterns of juvenile dispersal indicate that they typically cluster by establishing territories near conspecifics (Innes et al. unpublished data in Molles et al. 2008), hence population size where the carrying capacity may not have been reached yet (i.e., current surviving populations) would provide a reasonable proxy for the levels of social interaction birds are likely to experience within populations (c.f. population density).
As opportunities for social interaction vary across kōkako populations that differ in size, we hypothesized three general song responses when populations
impoverished song repertoires and increased song sharing as the availability and generation of novel song examples will be reduced relative to larger populations.
(2) During song bouts, birds in smaller populations should display lower song rates as well as lower switching rates and diversity of phrase types, as a result of interacting with a more restricted range of conspecifics. (3) Finally, we expected lower sequential entropy values in smaller populations due to the reduced
complexity of vocally interacting with fewer conspecifics.